1. Field of the Invention
This invention relates to a negatively chargeable toner used in recording processes which utilize electrophotography, electrostatic recording, electrostatic printing, toner jet system recording and the like, and also relates to an image-forming method and a process cartridge.
2. Related Background Art
In electrophotography, copied or printed images are commonly obtained by utilizing a photoconductive material and by forming an electrostatic latent image on a photosensitive member by various means, subsequently developing the electrostatic latent image by the use of a toner to form a toner image, and transferring the toner image to a transfer material such as paper as occasion calls, followed by fixing by the action of heat, pressure, heat-and-pressure or solvent vapor.
As developing systems applied in these electrophotographic processes, they are roughly classified into dry-process developing systems and wet-process developing systems. The former is further classified into a system making use of two-component developers and a system making use of one-component developers.
As toners used in such dry-process developing systems, particles of a binder resin in which a colorant has been dispersed and which has been pulverized to have particle diameters of approximately from 5 μm to 15 μm are used as a toner, for example. As magnetic toners which are one-component developers, those incorporated with magnetic fine particles as a colorant are used. In the case of the two-component developers, toners are also used in the form of a blend of a toner incorporated with carbon black or pigment as a colorant and carrier particles such as iron powder or magnetic ferrite particles.
In either developing system toners, toner particles must positively or negatively electrostatically be charged in accordance with the polarity of the electrostatically charged image to be developed.
To provide the toner particles with electric charges, the triboelectric chargeability of binder resins of toners may be utilized. In this case, however, the toner particles have so small charge quantity that the images formed by development may have a low image density and tend to fog, tending to be unsharp images.
Accordingly, in order to endow the toners with the desired triboelectric chargeability, dyes, pigments or polymeric compounds are added as charge control agents. As charge control agents for positive chargeability, nigrosine dyes, azine dyes, copper phthalocyanine pigments and quaternary ammonium salts are used. As charge control agents for negative chargeability, organometallic compounds of monoazo dyes and organometallic compounds of salicylic acid, naphthoic acid or dicarboxylic acid are used. However, such charge control agents stand colored in many cases, and may cause a problem in color reproducibility when used in color toners. Thus, chargeability control resins which are almost colorless or not so colored attract notice.
For example, Japanese Patent Application Laid-Open No. 63-184762 discloses a toner which has a binder resin comprised of a copolymer of styrene and/or α-methylstyrene with an acrylic or methacrylic alkyl ester and contains a copolymer of styrene and/or α-methylstyrene with 2-acrylamido-2-methylpropanesulfonic acid.
Japanese Patent Application Laid-Open No. 2-167565 also discloses a toner containing i) a chargeability control resin comprised of a sulfonic-acid-containing acrylamide monomer and a vinyl monomer and ii) a charge control agent other than this.
Japanese Patent Application Laid-Open No. 8-123096 discloses a toner containing i) a charge control agent containing an iron element and ii) a copolymer of a styrene/acrylic monomer with a sulfonic-acid-containing acrylamide monomer.
Japanese Patent Application Laid-Open No. 2000-305318 discloses a negatively chargeable toner containing a polymer having a sulfonic-acid group such as 2-acrylamido-2-methylpropanesulfonic acid.
When such polymeric compounds are used as charge control agents, it is difficult for them to be dispersed in toner's binder resins which are relatively hydrophobic, because the sulfonic-acid-group-containing monomer component such as 2-acrylamido-2-methylpropanesulfonoc acid is strongly hydrophilic. Especially in a running test where images are formed on many sheets in a high-temperature and high-humidity environment, such compounds tend to cause a decrease in image density and an increase in fog. According to studies made by the present inventors, in order to improve their initial charge characteristics of toners and their dispersibility in binder resins, the sulfonic-acid-group-containing monomer must be made more hydrophilic in molecular structure. Thus, it has not been sayable to be sufficient to merely somewhat change copolymerizable monomers.
Japanese Patent Applications Laid-Open Nos. 9-166887 and 2001-305793 disclose toners containing a copolymer with a styrene/acrylic monomer having a specific carboxylic-acid monomer (succinic-acid monohydroxyethyl methacrylate). When the above copolymer is used as a charge control agent, it is difficult to use it in toners because of its insufficient charge-providing performance. Hence, it has been necessary to use a conventionally known charge control agent in combination.
Japanese Patent Application Laid-Open No. 61-198249 also discloses a toner comprised of a binder resin having as a constituent unit a monomer containing an amino group. This binder resin, however, is a binder resin for positive charging which contains an alkylene group in the monomer at the same time, and is not a resin which can attain satisfactory charging performance when applied in negatively chargeable toners.